Inhibitor for nitriding processes



'so. as'toavoid the resultant brittleness thereof Patented Nov. 13, 1934 v 1,980,670 PATENT ore-r ce INHIBITORLFORLNITRIDING PROCESSES Harold'A; Eek-man, Oak Park, and Henry W.

Maack', Chicago, 111.,

as'signors to Crane 00.,

Chicago, 111.", a corporation oflllinois N0. Drawing.

Application June 20, .1931,

Serial No. 545,834-

5 Claims.

Thisinventionrelates to the. surface hardening of ferrous alloys by the nitriding process and moreparticularly, to, a means for preventing'the. hardening. of selected parts of, articles being treated by the nitridingprocess to. harden sur face layers.

In. the surface hardening of ferrous alloy articles it is often desirable to prevent. certain portions of the articles from being hardened,

asin" the case of screw threaded portions. or other parts which may be desiredv soft for subsequent machining or. forother reasons.

We are aware thatJmethodS have been used "and coatings supplied for selective hardening of steel articles where the hardening of the surface was obtained by a carburizing' process but'we have found by actual trial of all these known methods and coatings that theyare ineffectual.

'in preventing hardening obtained by nitriding processes due to the particular problems and conditions presented by the nitriding processes. We are also aware that it has'been proposed to tin portions of. the articles not desired to harden. This method however is very costly, requires personal skill and is not satisfactory at nitriding.

temperatures.

We have also conceived of a coating in the nature of a paint which may be appliedito. those portions which are desired to be. protected against hardening bynitriding. Insucha coati'ng, that is, a paint-like coating in liquid form,

embodying a metalpigmentin a vehicle carrier,

the. latter comprising. a binder together. with. a-

flux and solventfor thelatter, considerable difficulty is encountered in. providing: a binder havingsufficientbonding properties and in-prov-id+ ing a flux. particularly suitable. from a. commercial point of. view. This invention is in a broad sense an improvement upon the above method and particularly as to the physicalcharacteristics of the inhibitor. is. toprovide an. inhibitor to protect those por --tions of the surfaces of articles which are desired-to. be left relatively soft while the: adjacent unprotected portions are being hardenedby-the:

nitriding process.

As previously stated, the nitriding process; of hardening involves theuse of chemicals at temperatures and under conditions which prevent the. use of many substances as inhibitors which.

might otherwise be desirable. Because of the tendency of the substances to react chemically 'under. nitriding' conditions with the metal being treated or with the elements used in the-nitrid Its general: object esses tend .to cause decomposition of a greatmany substances which'u'nder high' temperatures volatilize and are carried out with the exhaust gases. Furthermore, it has been observed that: in nitriding furnaces equipped with fans' for" forced circulation of" the ammonia used for nitriding; small amounts" of the pigment maybe carried along with the fumes formed from the. binder or fi'ux probably in the form of aa fine. dust. This is'liable to cause soft'spotsto occur: in the otherwise hardened surface, providing" proper precautions are not taken: to* guard against the carrying off of 'the pigment: and the formation offumes from the binder and flux:

According to this invention, the partsto be protected, that" is remain untreated by: the. nitridingprocess, are covered with what: might be termed a stop-off or inhibitor or. protec:-- tive coating constituting'a covering: of acemem titious substance which is not d'etrimentally affected by the nitridingprocess; which: will withstand the temperatures and conditions, and. which will not affect the final suitability ofxthez part being treated. For this'purposeit has beenz. found thata cemeirt like substance of 2' a consistency that permits it" being applied readily asa; with a brush" is very desirable and very satis. factory.

However, it isobvious-that the mixture'may be applied'aswell by means of a swab, spray; dipping or other suitable means; and after: drying, the article may be subjected: to thenitrida ing process. By 'a cementitious substance: is. meant a material whichdue to chemical reaction has" the ability to set'andiharden or: form: a cement which holds a pigment suchas pow dered tin in place, the cement being-of such a character'that-it is not-subject-to decomposi-- tion" under nitriding conditions.

It has been found that a phosphate cement serves'the purpose very satisfactorily. The cementl may. beformed I byv mixing av vehicle containing phosphoric acidwith' a suitable pigment, the" pigment being either metal powderor metal pow--- d'er mixed with-non-metallic material. The ce-- ment is formed-bythereaction-of the phosphor ic acid on either the metal powder or the nonmetallic material or-both'. The settingand hardening of the cementitious material is of course dependent upon the nature and proportions of the vehicle and pigment and can: be governed so as to set slowly and-permit'thecementitious ma-- terial' being applied conveniently, as a paint. The metal of the protective coating alloys-with the steel surfaceand thuscauses-a-very tight-cf: fective bond of the cement to the surface being protected; Inasmuch as -the' surfaces to be protected are not as a rule suficiently clean to permit a perfect bonding of the cement with the will not volatilize and sublimate at nitriding,

' conditions and which may be obtained commercially in powdered form. To reduce the amount of metal required it has been found preferable to mix the metal in powdered form with a nonmetallic material, for example kieselguhr, silica, kaolin, or similar substances in powdered form.

A non-metallic material or filler may be chosen so as to aidin thechemical reaction to form a cement. The ability of the non-metallic material to aid in the formation of a cement is very important in mixtures where the metal powder is not affected by the phosphoric acid solution. The presence of. metallic oxides such as zinc oxide, lead oxide, tin oxide or other oxides has proven very effective in the setting or hardening of the cement.

Inasmuch as phosphoric acid reacts very rapidly, it has been found very desirable to employ phosphoric acid salts, for example, ammonium phosphate. usingammonium phosphate may vary from dilute to: very strong. concentrations although it has been found that a concentration within the range of 20% to is preferable.

As a flux, any substance or material having the ability to clean and cause the metal of the pigment to unite or closely bond with the surface of the steel alloy being nitrided may be used. Boric'acid has been found suitable as it does not volatilize at nitriding temperatures. Zinc chloridemay also be used but it is necessary however in this case to keep the vehicle acid in reaction to prevent precipitation of zinc compounds. The use-of an acid vehicle also facilitates the application of the inhibitor to the steel. Zinc chloride has also been found to contribute to the cementing properties of the inhibitor. The presence in small amounts of boric acid in a vehicle containing zinc chloride as a flux appears to improve the properties of the cement.

As a specific example, it has been found that a suitable protective. coating or inhibitor may be made from the following ingredients within the proportion ranges specified:

, Percent Ammonium phosphate in solution 30 to 40 Boric acid in water solution 3 to 10 Tin powder 35 to 90 Ground silica 5 to Ground kaolin 1 to 25 "One example of a satisfactory specific mixture is as follows:

Tin powder -1- '77 Ground kaolin 3 Ground silica 20 Where it is desired to use zinc chloride, the

The strength of the solution ingredients within the proportion following ranges have been found satisfactory:

Percent Ammonium phosphate in solution 20 to 30 Zinc chloride in a sufiiciently strong acid 30 solution, preferably hydrochloric acid to dissolve any precipitated zinc compound 1 to 7 Tinpowder 35 to 90 Ground silica 5 to 60 Ground kaolin 1to 25 Specific proportions in the following order have been found very satisfactory:

In solution:

Percent Ammonium phosphate 21 Zinc chloride 5 Boric acid 4' Muriatic acid 21 Water 49 In powder: 7 Percent Tin powder 70 Kaolin -Q 4 Silica V "26 d We claim:

1. A protective coating or inhibitor for use in a nitriding process comprising a cementitious 7 material consisting of 30% to 40% ammonium 0 phosphate in solution, 3% to 10% boric acid in water solution, and a pigment including 35% to 90% tin powder, 5% to 60% ground silica and 1% to 25% ground kaolin, the inhibitor deriving its hardening and setting characteristics from no the chemical reaction between the liquids and solids. I

2. A protective coating or inhibitor for use in a nitriding process comprising a cementitious material consisting of substantially 34% am- 15 monium phosphate and 8% boricacid in solution and a pigment of 77% tin powder, 3% ground kaolin and 20% ground silica, the inhibitor deriving its hardening and setting char acteristics from the chemical reaction between the liquids and solids. I

3. A protective coating or inhibitor for use in a nitriding process comprising a cementitious material consisting of 20% to 30% ammonium phosphate and 1% to 7% zinc chloride in acid' solution and a pigment of 35% to 90% tin powder, 5% to 60% ground silica and 1% to 25% ground kaolin, the inhibitor deriving-its hardening and setting characteristics from the chemical reaction between the liquids and solids.

4. A protective coating or inhibitor for use in a nitriding process'comprising a cementitious material consisting of 20% to 30% ammonium water, and a pigment of 70% tin powder, 4% 145 ground kaolin and 26% ground silica.

. HAROLD A. ECKMAN.

HENRY .W. MAACK. 

